A fuel injection apparatus for supplying fuel to an internal combustion engine which drives a vehicle without mounting no battery is driven by a power supply apparatus which generates a DC voltage using a magneto AC generator driven by the internal combustion engine as a power supply. When a fuel injection apparatus is operated using a magneto-generator driven by the internal combustion engine as a power supply, it is necessary to output a sufficiently high voltage from the power supply apparatus during low-speed rotation of the engine in order to improve a starting characteristic of the internal combustion engine. For this purpose, the number of windings of an armature coil of the magneto-generator may be increased, but increasing the number of windings of the armature coil reduces power generation output during a high-speed rotation of the engine. Decreasing the number of windings of the armature coil to improve power generation output during a high-speed rotation of the engine increases the rotational speed at which the fuel injection apparatus can operate normally, which deteriorates the starting characteristic of the engine.
Therefore, as described in Japanese Utility Model Registration No. 3024455, the applicant of the present invention has proposed a power supply apparatus which constitutes a full-wave rectifier circuit which rectifies output of a magneto AC generator with a bridge circuit comprised of a diode and a MOSFET (MOS type field effect transistor), controls ON/OFF of the MOSFET to thereby turn ON/OFF a current flowing from an armature coil of the generator through a rectifier circuit and obtain a stepped up DC voltage on the output side of the rectifier circuit.
The power supply apparatus disclosed in Japanese Utility Model Registration No. 3024455 is comprised of n (n: integer of 2 or greater) input terminals to which a single-phase or multi-phase AC voltage is input from a magneto AC generator driven by an internal combustion engine, m (m: integer of 2 or greater and not greater than n) MOSFETs with ones of drains and sources commonly connected and the others connected to m input terminals of the n input terminals, parasitic diodes formed between the drains and sources of the m MOSFETs and 2n−m rectification diodes provided so as to constitute together a diode bridge full-wave rectifier circuit which rectifies an AC voltage given to the n input terminals, an FET control section which controls MOSFETs with a reverse voltage applied to parasitic diodes between drains and sources out of the m MOSFETs when the rotational speed of the internal combustion engine is equal to or lower than a set value so as to turn ON/OFF at a frequency higher than the output frequency of the generator and a pair of output terminals to which the output voltage of the diode bridge full-wave rectifier circuit is applied, and a DC voltage obtained between the output terminals is given to the fuel injection apparatus of the internal combustion engine as a supply voltage.
The power supply apparatus disclosed in Japanese Utility Model Registration No. 3024455 is provided with a comparator which compares a potential of each input terminal of each phase, to which output terminals of each phase of the generator is connected, with a potential of other input terminal of other phase. The comparator compares the potentials of the input terminals of three phases, thereby detects whether a voltage applied between a drain and source of each MOSFET is in a positive half cycle period (half-cycle period during which a reverse voltage is applied to a parasitic diode of each MOSFET) or in a negative half-cycle period (half-cycle period during which a forward voltage is applied to the parasitic diode of each MOSFET) and gives a drive signal to turn ON/OFF each MOSFET between the gate and source of each MOSFET for a period during which a positive half-cycle voltage is applied between the drain and source of each MOSFET. Furthermore, for a period during which a forward voltage is applied to the parasitic diode of each MOSFET, the power supply apparatus is designed to reduce channel loss by giving a control voltage (voltage signal at a certain level) at an ON-level or higher between the gate and source of each MOSFET.
When the rotational speed of the engine is low and the voltage induced to the armature coil of the generator is low, the above described construction allows an intermittent current to flow from the armature coil through the MOSFET and allows the induced voltage to step up, thus making it possible to output a sufficiently high voltage from the power supply apparatus even when the rotational speed of the engine is low and cause the fuel injection apparatus to operate satisfactorily.
Furthermore, if a control voltage equal to or higher than the ON-level (level of the voltage given between the gate and source to turn on the MOSFET) is given between the gate and source of each MOSFET for a period during which a forward voltage is given to the parasitic diode formed between the drain and source of each MOSFET as described above, it is possible to reduce channel loss produced in the MOSFET and thereby improve the output during a low-speed rotation.
As described above, the conventional power supply apparatus is provided with a comparator which compares a potential of each input terminal of each phase with a potential of other input terminal of other phase to detect the polarity of the voltage applied between the drain and source of each MOSFET, and thereby requires an expensive comparator and cannot help but increase the cost. On the other hand, the conventional power supply apparatus disclosed in Japanese Utility Model Registration No. 3024455 requires the phase to be clearly distinguished when the output terminal of each phase of the generator is connected to the input terminal of the power supply apparatus, and therefore it is necessary to take measures to identify phases of output leads of the generator. For example, colors of output leads of the generator must be changed to identify phases of the output leads, which cause the cost of the generator to be increased.